Modern self-service stores are often equipped with a number of pay desks arranged in rows adjacent the exit from the store. To prevent the unauthorized removal of goods from the stores via the pay desk passages, these are at present usually provided with anti-theft detectors, which are disposed for co-action with anti-theft tags attached to the respective goods, e.g. in the form of a section of magnetic tape. Such installations usually operate according to the magnetic field principle, i.e. a magnetic alternating field is directed across the passage along which the customer is to pass. The alternating field is achieved by coil loops on either side, forming a portal in the passage. The coil loops are fed with an alternating current of given frequency, receiver coil loops being arranged in the respective portals and connected to detectors which react from the overtone oscillations generated by the applied alternating field when an anti-theft tag comes into the field.
With the aid of a conveyor belt outside the magnetic field, the goods are moved past the pay desk, where the tag is put into an inactive state in conjunction with reading the price code. On the other hand, the customer passes through the magnetic field, and possibly hidden goods provided with anti-theft tags are then detected such as to trigger an alarm.
In general, a plurality of coil systems are coupled together in the portals and are driven from a common alternating current source, all portal passages then having the same frequency in their alternating fields. This is a very economical and rational solution, but in practice certain disadvantages have been found to occur when the detector sensitivity must be taken to a very high level, e.g. for being able to detect active anti-theft tags in a shopping cart (which generally serves as a magnetically screening cage). It has thus been found that, due to random field effects, the degree of detection at a pay desk has been deteriorated by shopping carts, baby carriages etc. passing by an adjacent pay desk. Furthermore, this effect also causes false alarms in adjoining systems. This is of course quite unsatisfactory, and can create irritation, apart from there being difficulty in locating the origin of alarm.
In order to solve the problems mentioned, attempts have been made to arrange assorted plates and turbulent current-catching screens between the pay desks. However, it has been found that such arrangements, no matter how well they may be balanced and matched, have a deleterious effect on the sensitivity of the detectors.